As the demand for fuel increases worldwide, there is increasing interest in producing fuels and blending components from sources other than crude oil. Often referred to as a renewable source, these sources include, but are not limited to, plant oils such as corn, rapeseed, canola, soybean, microbial oils such as algal oils, animal fats such as inedible tallow, fish oils and various waste streams such as yellow and brown greases and sewage sludge. A common feature of these sources is that they are composed of glycerides and free fatty acids (FFA). Both triglycerides and the FFAs contain aliphatic carbon chains having from about 8 to about 24 carbon atoms. The aliphatic carbon chains in triglycerides or FFAs can be fully saturated, or mono, di or poly-unsaturated.
Hydroprocessing can include processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products. Hydrotreating is a process wherein hydrogen is contacted with hydrocarbons in the presence of hydrotreating catalysts which are primarily active for the removal of heteroatoms, such as sulfur, nitrogen, oxygen and metals from the hydrocarbon feedstock. In hydrotreating, hydrocarbons with double and triple bonds such as olefins may be saturated.
The production of hydrocarbon products in the diesel boiling range can be achieved by hydrotreating a biorenewable feedstock. A biorenewable feedstock can be hydroprocessed by hydrotreating followed by hydroisomerization to improve cold flow properties of product diesel. Hydroisomerization or dewaxing is a hydroprocessing process that increases the alkyl branching on a hydrocarbon backbone in the presence of hydrogen and hydroisomerization catalyst to improve cold flow properties of the hydrocarbon. Hydroisomerization includes hydrodewaxing herein.
Diesel fuel streams must meet cold flow property specifications particularly for winter fuel use. One cold flow property is “pour point” which is the temperature at which a hydrocarbon stream becomes semi-solid and loses its flow characteristics. A high pour point is generally associated with a higher normal paraffin content or a normal paraffin content comprising higher carbon number. Another cold flow property is “cloud point” which is the temperature below which wax in the hydrocarbon stream begins to form a cloudy appearance. The “cold filter plugging point” of diesel fuel is the temperature at which the presence of solidified waxes clogs fuel filters and injectors in engines. The wax also can accumulate on cold surfaces such as on a pipeline or heat exchanger tube and form an emulsion with water.
If co-processing bio-renewable feedstock in a distillate hydrotreating unit along with mineral feedstock, there is a challenge of meeting the cold flow property requirements of the finished product diesel because the diesel range product converted from bio-renewable feedstock consists largely of normal paraffins. The addition of a bed of hydrodewaxing or hydroisomerization catalyst in the hydrotreating reactor may not be effective to produce biorenewable diesel with adequate cold flow property requirements.
The hydrogenation of biorenewable feedstock results in CO2 and CO in the hydrotreated effluent. Carbon dioxide may be removed in an amine recycle gas scrubber. However, carbon monoxide concentration can only be reduced by purging a portion of the recycle gas. Since carbon monoxide inhibits the activity of hydrodewaxing or hydroisomerization catalyst, it may not be possible to place hydrodewaxing or hydroisomerization catalyst in the hydrotreating reactor, and it may not be possible to use hydrotreating recycle gas from the amine recycle gas scrubber as recycle gas for a separate hydrodewaxing or hydroisomerization reactor.
It would be desirable to provide a process and apparatus for the production of distillate hydrocarbons from a renewable feedstock that can meet cold flow property requirements applicable to diesel.